8000 Use _Alignof to query alignments in the struct module · Issue #132868 · python/cpython · GitHub
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skirpichev opened this issue Apr 24, 2025 · 2 comments
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Use _Alignof to query alignments in the struct module #132868

skirpichev opened this issue Apr 24, 2025 · 2 comments
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extension-modules C modules in the Modules dir type-feature A feature request or enhancement

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@skirpichev
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skirpichev commented Apr 24, 2025 •
edited by bedevere-app bot
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Feature or enhancement

Proposal:

The struct module uses pre-C11 hacks to determine alignment for types:

cpython/Modules/_struct.c

Lines 79 to 106 in 5808889

/* Define various structs to figure out the alignments of types */
typedef struct { char c; short x; } st_short;
typedef struct { char c; int x; } st_int;
typedef struct { char c; long x; } st_long;
typedef struct { char c; float x; } st_float;
typedef struct { char c; double x; } st_double;
#ifdef Py_HAVE_C_COMPLEX
typedef struct { char c; float complex x; } st_float_complex;
typedef struct { char c; double complex x; } st_double_complex;
#endif
typedef struct { char c; void *x; } st_void_p;
typedef struct { char c; size_t x; } st_size_t;
typedef struct { char c; _Bool x; } st_bool;
#define SHORT_ALIGN (sizeof(st_short) - sizeof(short))
#define INT_ALIGN (sizeof(st_int) - sizeof(int))
#define LONG_ALIGN (sizeof(st_long) - sizeof(long))
#define FLOAT_ALIGN (sizeof(st_float) - sizeof(float))
#define DOUBLE_ALIGN (sizeof(st_double) - sizeof(double))
#ifdef Py_HAVE_C_COMPLEX
# define FLOAT_COMPLEX_ALIGN (sizeof(st_float_complex) - sizeof(float complex))
# define DOUBLE_COMPLEX_ALIGN (sizeof(st_double_complex) - sizeof(double complex))
#endif
#define VOID_P_ALIGN (sizeof(st_void_p) - sizeof(void *))
#define SIZE_T_ALIGN (sizeof(st_size_t) - sizeof(size_t))
#define BOOL_ALIGN (sizeof(st_bool) - sizeof(_Bool))

Now we can use _Alignof operator, as C11-compatible compiler is a build requireent.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs
@skirpichev skirpichev added the type-feature A feature request or enhancement label Apr 24, 2025
@skirpichev skirpichev self-assigned this Apr 24, 2025
@skirpichev skirpichev mentioned this issue Apr 24, 2025
Merged
@picnixz picnixz added interpreter-core (Objects, Python, Grammar, and Parser dirs) extension-modules C modules in the Modules dir and removed interpreter-core (Objects, Python, Grammar, and Parser dirs) labels Apr 24, 2025
@picnixz
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picnixz commented Apr 24, 2025 •
edited
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Considering I did use _Alignof in my recent PR (#132428), I support this proposal.

skirpichev added a commit to skirpichev/cpython that referenced this issue Apr 24, 2025
@skirpichev
pythongh-132868: use _Alignof() operator in the struct module
10eb674
@bedevere-app bedevere-app bot mentioned this issue Apr 24, 2025
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@skirpichev
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PR is ready: #132872

picnixz pushed a commit that referenced this issue Apr 25, 2025
@skirpichev
gh-132868: use _Alignof() C11 operator in the struct module (#132872
ecd0373 
)
@picnixz picnixz closed this as completed Apr 25, 2025
@skirpichev skirpichev removed their assignment Apr 26, 2025
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